The present invention relates to thermal barrier coatings for components exposed to elevated temperatures and, more particularly, to thermal barrier coatings having reduced thermal conductivity by virtue of morphological coating features.
Thermal barrier coating systems of various types are well known in the gas turbine engine industry for protecting nickel-based and cobalt-based superalloy components, such as turbine blades and vanes, from oxidation and corrosion during engine operation.
One type of thermal barrier coating system involves depositing on the superalloy component (substrate) to be protected a bondcoat comprising an MCrAlY alloy overlay where M is iron, nickel, cobalt, or a combination thereof, oxidizing the bondcoat to form an alumina layer in-situ thereon, and then depositing a ceramic thermal barrier coating having columnar morphology on the alumina layer. Such a thermal barrier coating is described in U.S. Pat. Nos. 4,321,310 and 4,321,311.
Another type of thermal barrier coating system exemplified by U.S. Pat. No. 5,238,752 involves forming on the superalloy component (substrate) to be protected a bondcoat comprising nickel aluminide (NiAl) or platinum-modified nickel aluminide diffusion layer. The bondcoat is oxidized to form a thermally grown alumina layer in-situ thereon, and then a ceramic thermal barrier coating having columnar morphology is deposited on the alumina layer.
Murphy U.S. Pat. Nos. 5,716,720 and 5,856,027 involves forming on the superalloy component to be protected a bondcoat comprising a chemical vapor deposited platinum-modified diffusion aluminide coating having an outer additive layer comprising an intermediate Nixe2x80x94Al phase. The bondcoat is oxidized to form a thermally grown alumina layer in-situ thereon, and then a ceramic thermal barrier coating having columnar morphology is deposited on the alumina layer.
A widely used ceramic thermal barrier coating for aerospace applications to protect components, such as turbine blades, of the hot section of gas turbine engines comprises 7 weight % yttria stabilized zirconia (7YSZ). Two methods of applying this ceramic coating have been widely used. Electron beam physical vapor deposition (EBPVD) has been used to produce a coating columnar structure where the majority of coating porosity is located between relatively dense ceramic columns that extend generally perpendicular to the substrate/bondcoat.
Air plasma spraying also has been used to apply the 7YSZ ceramic coating in a manner to create about 10% by volume porosity in the as-deposited coating. This porosity is in the form of gaps between plasma xe2x80x9csplatxe2x80x9d layers and micro-cracking due to ceramic shrinkage. The thermal conductivity of as-manufactured plasma sprayed 7YSZ ceramic coatings generally is about 60% of that of the 7YSZ ceramic coatings applied by EBPVD.
An object of the present invention is to provide an improved thermal barrier coating and coating method wherein the ceramic coating has reduced thermal conductivity by virtue of morphological coating features.
The present invention provides a ceramic thermal barrier coating on a substrate wherein at least a layer portion of the coating comprises primary columnar grains extending transversely of a surface of the substrate and including integral secondary columnar grains extending laterally therefrom relative to a respective column axis. The secondary columnar grains typically extend from the primary columnar grains at an included acute angle of less than 90 degrees relative to the column axis of the primary columnar grains. The coating structure unexpectedly exhibits reduced thermal conductivity as compared to a conventional thermal barrier coating.
The thermal barrier coating can comprise multiple layers wherein one of the layers comprises a coating structure pursuant to the invention. For example only, a thermal barrier coating can include an inner layer adjacent to a surface of the substrate having a conventional columnar grain structure and an outer layer pursuant to the invention having primary columnar grains with secondary columnar grains extending laterally therefrom.
The invention further provides an EBPVD method of making a thermal barrier coating wherein deposition of ceramic material on the substrate surface is controlled to grow the primary columnar grains having secondary columnar grains growing laterally therefrom spaced along the length of the primary columnar grains during coating deposition.
Advantages and objects of the invention will become more readily apparent from the following detailed description taken with the following drawings.